Process and apparatus for dielectric heating



Jan. 26, 1960 w. A. SCHATTLER ET AL 2,922,865

PROCESS AND APPARATUS FOR DIELECTRIC HEATING Filed May 26, 1958 AFTO NE)United States Patent PROCESS AND APPARATUS FOR DIELECTRIC HEATINGWilbul'n A. Schattler, Warren, and Ralph M. Stallard, Utica, Mich.,assignors to General Motors Corporation, Detroit, Mich., a corporationof Delaware Application May 26, 1958, Serial No. 737,812 Claims. (Cl.21910.41)

This invention relates to an improved process and apparatus fordielectric heating and embossing.

-In the process of dielectric heating, the material to be processed ispressed between two electrodes, generally in the form of metal plates,while high frequency alternating voltage is applied to generate heatwithin the material. For dielectric embossing, the surface of one of theelectrodes is contoured with the design desired to be imparted to thematerial during the dielectric heating and pressing step. The processmay be used, for example, to manufacture decoratively embossedautomobile upholstery comprising a trim material positioned on a fibrousor foam pad which is made of or impregnated with a heatf usible plasticand is supported on a fiberboard backing. Upon operation of thedielectric press an embossed pattern is produced wherein the trimmaterial is bonded to the backing through the pad, the plastic in thepad along the embossed lines having been melted and cured, thus servingas the bonding adhesive. In practice, the frequency used for thedielectric heating is in the range of 2 to 200 megacycles per second.Copending United States patent applications Serial No. 538,914, filedOctober 6, 1955, in the names of Peter P. Dusina, Jr., and Ralph M.Stallard; Serial No. 643,654, filed March 26, 1957, in the names of MingC. Hsu and Ralph M. Stallard; Serial No. 702,553, filed December 13,1957, in the name of Ralph M. Stallard; and Serial No. 651,356, filedApril 8, 1957, in'the names of Ming C. Hsu and Ralph M. Stallard; all ofwhich are assigned tothe assignee of the present invention, relate tosuch processes. I It is well known that the amount of heat generated inthe material being dielectrically heated is dependent upon the highfrequency voltage across the material. Therefore it is desirable to usea high voltage in order to obtain the maximum heating etiect in theshortest time possible. The limiting factor on the voltage, however, isthe dielectric strength of the materials between the electrodes, and oneof the serious problems which has been encountered in dielectric heatingand embossing operations is that of arcing with resultant damage to thematerial being processed, and sometimes also to the electrodes.Assurance against arcing may, ofcourse, be accomplished by using a lowvoltage; however, this is unsatisfactory since it reduces the speed andover-all efficiency of the process.

It is an object of the present invention to provide an improved processand apparatus for fast efiicient dielectric heating. More specifically,an object of the invention is to provide an improved dielectric heatingand embossing process and apparatus Which substantially eliminates thehazard of injury to the apparatus and to the material being processeddue to arcing.

These and other objects of our invention are achieved by a layer of aninsulating material of hereinafterspecified composition which ispositioned between at least one of the electrodes and the material beingprocessed. The composition of the insulating material is such as toSubstantially eliminate the hazard of arcing, and with Patented Jan. 26,1960 2. this hazard eliminated'higher electrical energies may be appliedto thereby add speed and increased efficiency to the process.

Other objects and advantages of the invention will be obvious from thefollowing detailed description made with reference to the accompanyingdrawing which shows a sectional view of a typical trim assemblypositioned in a dielectric press preparatory to embossment in accordancewith the invention, the view of the apparatus being generally schematic.

Referring now to the drawing, there is shown a dielectric embossingpress having upper and lower electrodes 2 and 4, respectively, in serieswith a high frequency generator 5. Between the electrodes is a laminatedtrim assembly comprising a sheet of plastic or similar trim material 8positioned on a layer 10 of sponge rubber and having a paper or clothbacking sheet 12 secured thereto. The upper electrode 2 is formed toserve as an embossing die by way of a plurality of embossing blades 6thereon which are arranged to impart the desired depressed pattern ordesign to the trim assembly by application of pressure and heat upon theclosing of the press and actuation of the high frequency generator. Itwill be understood that the precise composition, form and structure ofthe material being embossed forms no part of the present invention, theabove example being given merely for purposes of illustration.

In accordance with the present invention there is interposed between oneof the two electrodes and the material being embossed, a layer 14 ofinsulating material consisting essentially of from about 60% to bariumtitanate and the remainder a binder of heat-resistant organic polymer.The thickness of the insulating layer will, of course, depend on theamount of insulation desired and on the exact composition selected;generally, for optimum advantage, the thickness will be in the range of& inch to /2 inch. Where, as in the embodiment illustrated, only one ofthe electrodes is formed with embossing blades and the other electrodeis flat, it is preferable that the insulating layer be positioneddirectly over the surface of the fiat electrode as shown. It may eitherbe attached to the electrode by suitable clamps or the like, or may bemerely placed over the electrode prior to or simultaneously withinsertion of the material to be embossed.

The barium titanate used may be commercial grade and may show onanalysis minor amounts of inert or nondeleterious ceramic ingredients orimpurities such as zirconia, silica, iron oxide and the like. Likewise,the barium titanate may contain either baria or titania in slight excessof that required on a stoichiometric basis. The following analyses oftwo different commercially available barium titanates will serve toillustrate:

Percent by weight BaO 64.58 TiOz 28.99 ZrO 6.20

BaO 68.50 T10 32.08 F2O3 .07

' dimethylsiloxane-diyinylsiloxane copolymen cured by a tics and highheat resistance.

The preferred composition for the insulating layer 14 consists of fromabout 70% to 75% barium titanate and the remainder binder, preferablycured silicone rub- 4 invention is that it simplifies the manufacture ofthe electrodes, in the form of matched dies, used to press the clothsections together. That is, by way of the dielectric layer of thisinvention, minor inaccuracies in the spacing between the matched dieelectrodes may be tolerated without the hazard of arcing which mightotherwise occur. Thus, though'the detailed description has been withreference to a particular embodiment thereof, it will be understood thatother embodiments and modifications maybe used all within the full andintended scope of the claims which follow. 7

We claim:

1. A dielectric heating apparatus comprising a pair of electrodesconnected in series to a source of high freber. The following exampleswill serve to more. fully illustrate (1) One hundred parts (by weight)polydimethyl siloxanedi-vinylsiloxane gum, about sixteen partsditertiary butyl peroxide catalyst and 270 parts barium titanate arethoroughly milled together, pressed or calendered v to a sheet of A inchthickness and cured'for about 15 minutes at 340 F. in a press and thenfor an additional 24 hours at 480 F. in an oven. The resultingrubbermaterial has a dielectric constant of about 11 and a power factorof about .0029 at 14 me. s.

(2) Same as Example 1 except that 340 parts by weight barium titanateare used instead of 270 parts. With this increase inbarium titanatecontent, the resulting material has a dielectric constant of 14 and apower factor of .0027 at 14 me. s. Y

The most important advantageous characteristics of the preferredcompositions are a high dielectric constant and a low power factor, theformer in all instances exceeding about 6 and the latter not being inexcess of about .01. stray field adjacent to the embossing electrodesand thereby provides highly increased assurance against arcing even atrelatively high voltages on the order of 2000 volts RF and higher. Thelow power factor results in a minimum heat loss in the layer 14. Thus, arelatively short heat cycle may be used.

To practice the process of the invention, the insulating layer 14isattached or placed .over one of the electrodes of the dielectric pressand the materials to be processed are brought together with a pressureon the order'of 200 to 800 p.s.i. The embossing cycle generallycomprises two phases from the time standpoint, the time during whichdielectric heating takes place (the heat cycle) and the time thatpressure is maintained on the embossed assembly after the heating isaccomplished (soak time). A heat cycle of from 6 to 60 seconds at about13 megacycles per second, 2500 volts, and a hold time'of up to 10seconds are usually satisfactory. The precise times necessary foroptimum results will depend on the exact frequency and voltage used, thematerials being embossed, the nature of the pattern, etc. The peaktemperature reached during the heat cycle may be from 250 F. to 350 F.,a temperature of 325 F. being typical. After the soak time is completed,the embossed assembly is removed and anew assembly inserted. Theinsulating layermay, of course, be used over and over again, theexcellent wear characteristics of silicone rubber providing anexceptionally long, useful life.

While the invention has been described most specifically with referenceto dielectric embossing, it will be understood that it may also be usedto great advantage in other dielectric heating processes. For example,it has proven itself highly successful for dielectric bonding of clothto cloth with heat-fusible bonding material in the manufacture ofautomobile convertible tops. In this embodiment in particular, one ofthe advantages of the The high dielectric constant minimizes the quencyelectrical energy, the surface of at least one of said electrodes beingcovered by a layer of dielectric material consisting essentially of from60% to 90% by weight barium'titanate and the remainder silicone rubber.

2. A dielectric heating apparatus comprising a pair of electrodesconnected in series to a source of high frequency electrical energy, thesurface of at least one of said electrodes being covered by a layer ofdielectric material consisting essentially of from 70% to 75 by weightbarium titanate and the remainder silicone rubber.

3. A dielectric heating apparatus comprising a pair of electrodesconnected in series to a source of high frequency electrical energy andmeans for moving one of said electrodes toward the other so as to pressthe material to be heated therebetween, the surface of at least one ofsaid electrodes being covered by a layer of dielec- V tric materialconsisting essentially of from 60% to 90% by weight bariumrtitanate andthe remainder a resilient organic polymeric binder for said bariumtitanate.

4. A dielectric heating apparatus comprising a pair of electrodesconnected in series to a source of high frerubber.

' inserted between the electrodes, after which the electrodes 5. Adielectric embossing apparatus comprising a bottom flat electrode and atop embossing electrode connected in series to a source of highfrequency electrical energy, means for moving said electrodes togetherso as to press the material to be embossed therebetween, the,

energy, the improvement which comprises positioning between at least oneof said electrodes and the material to be heated a layer of dielectricmaterial consisting essentially of from 60% to by weight barium titanateand the remainder silicone rubber.

7. In a dielectric heating process wherein the material to be heated ispositioned between two electrodes connected in series to a source ofhigh frequency electrical energy, the improvement which comprisespositioning between at least one ofsaid electrodes and the material tobe heated a layer of dielectric material consisting essentially of from70% to 75% by weight barium titanate and the remainder silicone rubber.

8. In a dielectric embossing process the step of pressing the materialto be embossed between a flat electrode and an embossing electrodeconnected in series to a source 9. In a dielectric embossing process thestep of pressing with from 200 to 800 p.s,i. the material to be embossedbetween a fiat electrode and 'an embossing electrode connected in seriesto a source of high frequency electrical energy while having positionedbetween said flat electrode and the material being embossed a layer ofinch to /2 inch thickness of dielectric material consisting essentiallyof from 70% to 75% by weight barium titanate and the remainder siliconerubber.

10. In a dielectric heating process the step of pressing the material tobe heated between two electrodes connected in series to a source of atleast 2000 volts RF while having positioned between at least one of saidelectrodes and the material being heated a layer of dielectric materialhaving a dielectric constant of at least 6 and a power factor notexceeding .01 and consisting essentially of from 60 to 90% by Weightbarium titanate and the remainder a resilient heat resistant binder forsaid barium titanate.

References Cited in the file of this patent UNITED STATES PATENTS 102,412,982 Hart Dec. 24, 1946 2,667,437 Zonbek Ian. 26, 1954 2,734,982Gillespie Feb. 14, 1956 2,830,162 Copson et a1. Apr. 8, 1958

1. A DIELECTRIC HEATING APPARATUS COMPRISING A PAIR OF ELECTRODESCONNECTED IN SERIES TO A SOURCE OF HIGH FREQUENCY ELECTRICAL ENERGY, THESURFACE OF AT LEAST ONE OF SAID ELECTRODES BEING COVERED BY A LAYER OFDIELECTRIC MATERIAL CONSISTING ESSENTIALLY OF FROM 60% TO 90% BY WEIGHTBARIUM TITANATE AND THE REMAINDER SILICONE RUBBER.